1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine.
2. Description of the Prior Art
In the image forming apparatus such as the electrophotographic copying machine, there is used a photosensitive member which is prepared by forming a photoconductive layer of selenium-tellurium or amorphous silicon on a conductive substrate, and there are repeated cycles each of which includes the steps of charging all over surface of the photosensitive member, subjecting the charged photoconductive member to an image forming exposure to form an electrostatic image, developing the electrostatic image with toner to convert it into a toner image, and transferring the toner image attained to a transfer material such as paper to attain an image record. After the transfer of the toner image, the photosensitive member is cleared for reuse by removing a residual toner by means of a cleaning means. However, the charges are still left on or in surface of the photosensitive member so that they have to be removed before the photosensitive member is used again.
In order to neutralize the charges on the surface of the photosensitive member, there is currently adopted a method of exposing all over the surface of the photosensitive member. If the exposures are repeated, however, there appears a phenomenon that the charged potential of the photosensitive member is dropped by the influences of the repetition of the whole-surface exposures.
In the electrophotographic copying machine of the prior art such as a copying machine using a photosensitive member of selenium-tellurium, this photosensitive member can have its charge generating layer of selenium-tellurium of a relatively large thickness (e.g., about 60 microns) so that it can be charged at a high level of 700 to 1,000 V. The refer, even with the drop (.DELTA.V: usually equal to or lower than 70 V) of the charged potential during the repeated uses, the resultant changing rate of the charged potential is relatively low.
In the photosensitive member made of amorphous silicon (which will be shortly referred to as an "a-Si"), however, the a-Si charge generating layer to be formed is usually limited to a small thickness, e.g., 15 to 30 microns by the problems of its film forming technique or the mobility of the charge carriers (or shortly "carriers"). As a result, the potential to be able to charged (or shortly "charged potential") is about 300 to 600 V at the highest so that the changing rate of the charged potential is made liable to take a large value due to the dropping (.DELTA.V) of the charged potential. In order to form an image of high quality, therefore, it is indispensable to hold the dynamic range of the developing bias wide for the development and to stabilize the charged potential during the repeated uses. Especially in the a-Si photosensitive member, moreover, it is necessary to consider countermeasures for preventing the phenomenon called the "ghost" which is based upon fatigues due to the optical irradiation. This ghost is a phenomenon that the fatigues of the photosensitive member are made locally different or advanced by the ununiformity of the optical irradiation to leave a negative or positive image even during a subsequent copy operation so that a desired image cannot be attained (for example, the aforementioned left image appears with a high density in a half-tone image).
Incidentally, there are present in the prior art a variety of techniques using the a-Si photosensitive member, all of which have failed to satisfy the aforementioned requirements. In Japanese Patent Laid-Open No. 58-62659, for example, there is disclosed a technique in which the photosensitive member is irradiated with a ray of short wavelength lower than 600 nm as an optical ray for exposures and/or charge neutralizations. However, we found that the ghost is made liable to occur by the wavelength component of 550 nm or shorter of that short-wavelength ray and that the wavelength component of 550 to 600 nm does not always improve the repetition characteristics (i.e., the changing rate of the charged potential for the repeated uses). In the above-specified Laid-Open, moreover, there is also disclosed a concept that the aforementioned ray may contain a component of the longer-wavelength than 600 nm having an energy distribution ratio of 30% or smaller. As tne ratio of the longer-wavelength component, is too small, it is impossible to expect improvements in the repetition characteristics.